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Ocean Acidification: Climate Change’s Evil Twin!?

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This week our volunteers are learning about ocean acidification. Using just red cabbage juice, saltwater and a straw we demonstrated how Carbon Dioxide (CO2) increases the acidity of sea water.

Experiment Kit: Red Cabbage, Salt water, Plastic cups and Straws.(Learn how to do this experiment in our Biota in a Box booklet)

The red cabbage juice acts like Universal Indicator or Litmus paper; changing colour as the solution changes in pH.

Our lovely V Factor volunteers blowing bubbles!

By blowing into the solution of cabbage juice and ‘seawater’ the volunteers were exhaling CO2 into the solution.

As you can see the solution changed from blue (slightly alkali) to purple (slightly acidic).

Control (original mixture) and treatment (mixture which has had bubbles of CO2 blown into it)

As the amount of CO2 in our atmosphere increases, so too does the amount of CO2 dissolved in our oceans. Our experiment showed that when CO2 becomes dissolved in sea water it decreases its pH (making it more acidic). This is because the dissolved carbon dioxide forms carbonic acid which dissociates into hydrogen ions, H+, and bicarbonate ions, the bicarbonate ions then dissociate into more hydrogen ions and carbonate ions. The acidity of a solution increases with the concentration of hydrogen ions (see diagram below).1

Figure 1: Diagram showing how atmospheric carbon dioxide dissolves in water leading to ocean acidification 1

Since the industrial revolution the amount of CO2 in the atmosphere has increased. The graph below shows between 1960-2015 atmospheric CO2 has increased from approximately 320ppm to 400ppm.2 As a result the Current oceanic pH is around 8.2,3 slightly alkali, a drop of 0.1 since the industrial revolution (although this doesn’t sound like a lot remember the pH scale range is 0-14). According to IPCC (Intergovernmental Panel on Climate Change) projections, the amount of CO2 in our atmosphere could increase to 950ppm by 2100,4 potentially causing the pH of our oceans to decrease to 7.9.3

Bryozoans have calcium carbonate skeletons. Anyone who has used vinegar (acid) to remove limescale (calcium carbonate) will have a good idea what will happen to their skeletons if the oceans acidity continues to increase. Not only will ocean acidification corrode their skeletons but it also appears to cause changes in their mineralogy and decreased survival. Arctic species are likely to suffer the worst effects because cold water can dissolve larger quantities of CO2.

Calcium carbonate skeleton of an encrusting Bryozoan

Research published last week by an international team of scientists, including our colleague Paul Taylor, found that bryozoan colonies (Calpensia nobilis ) transplanted to areas of higher ocean acidity (near CO2 emitting volcanic vents in the Mediterranean sea) had slower or disrupted growth and their skeletons were slightly degraded. The levels of acidification were similar to those predicted for the year 2100. One a more positive note the researchers did find evidence that the changes in growth could in fact have been caused by adaption to the less than ideal conditions.5

Ocean acidifcation won’t just affect Bryozoans. It will have wider implications for marine ecosystems and humans who rely on the sea’s natural resources:

Declines in marine organisms with CaCO3 exoskeletons or shells. e.g. crustaceans and molluscs and corals (bleaching).

Loss of habitat (reefs)

Declines in larger marine animals who feed on affected species.

Loss of revenue from commercial fishing and tourism.

If you would like to learn more about ocean acidifcation check out the list of references below or come and visit our Volunteer Leaders outside the Specimen Preparation Area in the Natural History Museum every Thursday 11.00-13.00 and 14.00-16.00.